Spin-coatable liquid for formation of high purity nanotube films

ABSTRACT

Certain spin-coatable liquids and application techniques are described, which can be used to form nanotube films or fabrics of controlled properties. A spin-coatable liquid for formation of a nanotube film includes a liquid medium containing a controlled concentration of purified nanotubes, wherein the controlled concentration is sufficient to form a nanotube fabric or film of preselected density and uniformity, and wherein the spin-coatable liquid comprises less than 1×10 18  atoms/cm 3  of metal impurities. The spin-coatable liquid is substantially free of particle impurities having a diameter of greater than about 500 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §121, of the filingdate of U.S. patent application Ser. No. 10/860,334, filed Jun. 3, 2004,which claims the benefit of the filing date of U.S. patent applicationNo. 60/501,033, filed Sep. 8, 2003, both of which are incorporated byreference herein in its entirety.

This application is related to the following applications, all of whichare assigned to the assignee of this application, and all of which areincorporated by reference in their entirety: Nanotube Films and Articles(U.S. Pat. No. 6,706,402) filed Apr. 23, 2002; and Methods of NanotubeFilms and Articles (U.S. patent application Ser. No. 10/128,117) filedApr. 23, 2002.

BACKGROUND

1. Technical Field

This invention describes spin-coatable liquids for use in thepreparation of nanotube films. Such liquids are used in creating filmsand fabrics of nanotubes or mixtures of nanotubes and other materials ona variety of substrates including silicon, plastics, paper and othermaterials. In particular, the invention describes spin-coatable liquidscontaining nanotubes for use in electronics fabrication processes.Furthermore, the spin-coatable liquids meet or exceed specifications fora semiconductor fabrication facility, including a class 1 environment.

2. Discussion of Related Art

Nanotubes are useful for many applications; due to their electricalproperties nanotubes may be used as conducting and semi-conductingelements in numerous electronic elements. Single walled carbon nanotubes(SWNTs) have emerged in the last decade as advanced materials exhibitinginteresting electrical, mechanical and optical properties. However, theinclusion or incorporation of the SWNT as part of standardmicroelectronic fabrication process has faced challenges due to a lackof a readily available application method compatible with existingsemiconductor equipment and tools and meeting the stringent materialsstandards required in the electronic fabrication process. Standards forsuch a method include, but are not limited to, non-toxicity,non-flammability, ready availability in CMOS or electronics grades,substantially free from suspended particles (including but not limitedto submicro- and nano-scale particles and aggregates), and compatiblewith spin coating tracks and other tools currently used by thesemiconductor industry.

Individual nanotubes may be used as conducting elements, e.g. as achannel in a transistor, however the placement of millions of catalystparticles and the growth of millions of properly aligned nanotubes ofspecific length presents serious challenges. U.S. Pat. Nos. 6,643,165and 6,574,130 describe electromechanical switches using flexiblenanotube-based fabrics (nanofabrics) derived from solution-phasecoatings of nanotubes in which the nanotubes first are grown, thenbrought into solution, and applied to substrates at ambienttemperatures. Nanotubes may be derivatized in order to facilitatebringing the tubes into solution, however in uses where pristinenanotubes are necessary, it is often difficult to remove thederivatizing agent. Even when removal of the derivatizing agent is notdifficult, such removal is an added, time-consuming step.

There have been few attempts to disperse SWNTs in aqueous andnon-aqueous solvents. Chen et al. first reported solubilization ofshortened, end-functionalized SWNTs in solvents such as chloroform,dichloromethane, orthodichlorobenzene (ODCB), CS2, dimethyl formamide(DMF) and tetrahydrofuran (THF). See, “Solution Properties ofSingle-Walled Nanotubes”, Science 1998, 282, 95-98. Ausman et al.reported the use of SWNTs solutions using sonication. The solvents usedwere N-methylpyrrolidone (NMP), DMF, hexamethylphosphoramide,cyclopentanone, tetramethylene sulfoxide and ε-caprolactone (listed indecreasing order of carbon nanotube salvation). Ausman at el. generallyconclude that solvents with good Lewis basicity (i.e., availability of afree electron pair without hydrogen donors) are good solvents for SWNTs.See, “Organic Solvent Dispersions of Single-Walled Carbon Nanotubes:Toward Solutions of Pristine Nanotubes”, J. Phys. Chem. B 2000, 104,8911. Other early approaches involved the fluorination or sidewallcovalent derivatization of SWNTs with aliphatic and aromatic moieties toimprove nanotube solubility. See, e.g., E. T. Mickelson et al.,“Solvation of Fluorinated Single-Wall Carbon Nanotubes in AlcoholSolvents”, J. Phys. Chem. B 1999, 103, 4318-4322.

Full-length soluble SWNTs can be prepared by ionic functionalization ofthe SWNT ends dissolved in THF and DMF. See, Chen et al., “Dissolutionof Full-Length Single-Walled Carbon Nanotubes”, J. Phys. Chem. B 2001,105, 2525-2528 and J. L. Bahr et al Chem. Comm. 2001, 193-194. Chen etal. used HiPCO™ as-prepared (AP)-SWNTs and studied a wide range ofsolvents. (HiPCO™ is a trademark of Rice University for SWNTs preparedunder high pressure carbon monoxide decomposition). The solutions weremade using sonication.

Bahr et al. (“Dissolution Of Small Diameter Single-Wall Carbon NanotubesIn Organic Solvents?”, Chem. Commun., 2001, 193-194) reported the mostfavorable solvation results using ODCB, followed by chloroform,methylnaphthalene, bromomethylnaphthalene, NMP and DMF as solvents.Subsequent work has shown that good solubility of AP-SWNT in ODCB is dueto sonication-induced polymerization of ODCB, which then wraps aroundSWNTs, essentially producing soluble polymer wrapped (PW)-SWNTs. SeeNiyogi et al., “Ultrasonic Dispersions of Single-Walled CarbonNanotubes”, J. Phys. Chem. B 2003, 107, 8799-8804. Polymer wrappingusually affects sheet resistance of the SWNT network and may not beappropriate for electronic applications where low sheet resistance isdesired. See, e.g., A. Star et al., “Preparation and Properties ofPolymer-Wrapped Single-Walled Carbon Nanotubes”, Angew. Chem. Int. Ed.2001, 40, 1721-1725 and M. J. O'Connell et al., “ReversibleWater-Solubilization Of Single-Walled Carbon Nanotubes By PolymerWrapping”, Chem. Phys. Lett. 2001, 342, 265-271.

While these approaches were successful in solubilizing the SWNTs in avariety of organic solvents to practically relevant levels, all suchattempts resulted in the depletion of the π electrons that are essentialto retain interesting electrical and optical properties of nanotubes.Other earlier attempts involve the use of cationic, anionic or non-ionicsurfactants to disperse the SWNT in aqueous and non-aqueous systems.See, Matarredona et al., “Dispersion of Single-Walled Carbon Nanotubesin Aqueous Solutions of the Anionic Surfactant”, J. Phys. Chem. B 2003,107, 13357-13367. While this type of approach has helped to retain theelectrical conductivity and optical properties of the SWNTs, most suchmethods leave halogens or alkali metals or polymeric residues, whichtend to severely hamper any meaningful use in microelectronicfabrication facilities.

There is a need for a method of solvating or dispensing nanotubes insolvents for use in electronics applications. There remains a furtherneed for methods that meet the criteria outlined above for low toxicity,purity, cleanliness, ease of handling and scalability.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to spin-coatable liquidsfor formation of high purity nanotube films.

According to one aspect of the present invention, a composition ofnanotubes for use in an electronics fabrication process includes aliquid medium containing a plurality of nanotubes pretreated to reducethe level of metal and particulate impurities to a preselected level.The solvents are present at commercially meaningful levels, e.g., thenanotubes are at a concentration of greater than 1 mg/L. The nanotubesare homogeneously distributed in the liquid medium without substantialprecipitation or flocculation.

In one aspect of the present invention, a nanotube composition includesa stable distribution of nanotubes in a liquid medium and issubstantially free of particulate and metallic impurities. The level ofparticulate and metallic impurities is commensurate with preselectedfabrication requirements.

In one aspect of the invention, a spin-coatable liquid for formation ofa nanotube film is provided including a liquid medium containing acontrolled concentration of purified nanotubes, wherein the controlledconcentration is sufficient to form a nanotube fabric or film ofpreselected density and uniformity, and wherein the spin-coatable liquidcomprises less than 1×10¹⁸ atoms/cm³ of metallic impurities.

In another aspect of the invention a spin-coatable liquid for formationof a nanotube film is provided including a liquid medium containing adistribution of nanotubes, wherein the nanotubes remain separate fromone another without precipitation or flocculation for a time sufficientto apply the spin-coatable liquid to a surface, and wherein thespin-coatable liquid is substantially free of particle impurities havinga diameter of greater than about 500 nm.

In another aspect of the invention, a fullerene composition includes aliquid medium containing a distribution of fullerenes, wherein thefullerenes remain separate from one another without precipitation orflocculation for a time sufficient to apply the fullerene composition toa surface, and wherein the composition comprises less than 1×10¹⁸atoms/cm³ of metal impurities.

The fabrication processes can have varying requirements for solvent andraw material composition and purity. According to one aspect of thepresent invention, spin-coatable liquids containing fullerenes ornanotubes of varying composition and purity are provided for use inthese fabrication processes having varying processing specifications andenvironmental requirements.

According to one aspect of the present invention, methods andcompositions for creating nanotube compositions for use in fabricationfacilities having high standards of non-toxicity and purity areprovided. Such processes include semiconductor fabrication processes,for example, CMOS and advanced logic and memory fabrications. Suchfabrication processes may produce devices having fine features, e.g.,≦250 nm.

According to other aspects of the present invention, the nanotubecompositions are of a purity that is suitable for use in electronicsfabrication facilities having less stringent standards for chemicalcomposition and purity. Such processes include, for example,interconnect fabrication and fabrication of chemical and biologicalsensors.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described with reference to the Drawing, which ispresented for the purpose of illustration only and which is not intendedto be limiting of the invention.

FIG. 1 illustrates a typical scanning electron micrograph (SEM) of anunpurified nanotube fabric; and

FIG. 2 illustrates a typical SEM image of a purified nanotube fabric.

DETAILED DESCRIPTION OF THE INVENTION

Nanotubes have been the focus of intense research efforts into thedevelopment of applications that take advantage of their electronic,biological, and/or material properties. In one or more embodiments, aspin-coatable liquid containing a controlled concentration of purifiednanotubes is prepared in a liquid medium. The spin-coatable liquid maybe used to create nanotube films and fabrics of substantially uniformporosity. Certain embodiments provide spin-coatable liquids having apurity level that is commensurate with the intended application. Otherapplications provide spin-coatable liquids meeting or exceedingspecifications for class 1 semiconductor fabrication.

In one or more embodiments, a nanotube composition includes a liquidmedium containing a mixture of single-walled or multi-walled nanotubesthat is stable enough for certain intended applications, such as spincoating in a class 1 production facility. The nanotubes in the nanotubecomposition remain suspended, dispersed, solvated or mixed in a liquidmedium without substantial precipitation, flocculation or any othermacroscopic interaction that would interfere with the ability to applythe nanotube solution to a substrate and form a uniform porosity. Ifthere were significant precipitation or aggregation of the nanotubes,the nanotubes would clump together and form non-uniform films, whichwould be undesirable. The nature by which the nanotubes interact withthe solvent to form a stable composition is not limited. Thus, forexample, the nanotubes may be suspended or dispersed in the solvent orthey may be solvated or solubilized in the solvent. The stable nanotubecomposition typically forms a homogeneous distribution of nanotubes inthe solvent.

At the present time, it is desirable that the nanotubes remaindistributed in the solvent medium without substantial precipitation,flocculation or other macroscopic interaction, for at least one hour, orfor at least 24 hours, or even for at least one week. Substantialprecipitation and flocculation and the like can be detected by a varietyof methods. Precipitates and aggregates can be detected by visualinspection. Alternatively, precipitation or flocculation can be detectedby analytical techniques, such light scattering or absorbance, or byobservation of nanotubes once they are deposited on a substrate from thenanotube solution. A stable nanotube composition can exhibit prolongedsuspension (typically several weeks to few months) of the SWNT in themedium with little or no detectable change in the scattered lightintensity, or absorbance at a given wavelength, or viscosity.

Light scattering is measured using a monochromatic beam of lighttraveling through the solution. A change of light scattering intensityover time is recorded usually by a detector placed normal to the beamdirection or from multiple detectors placed at various angles includingthe right angle. Another indicator especially at low concentrations ofSWNT is the fall in absorbance (at a given wavelength) as function oftime. For higher concentrations of the solution, between the semidiluteand nematic regimes, precipitation of individually suspended tubes leadsto a noticeable fall in the viscosity of the suspension. Other methodsof determining the stability of a nanotube composition for its intendedpurpose will be apparent to those of skill in the art.

The nanotubes used in one or more embodiments of the present inventionmay be single walled nanotubes or multi-walled nanotubes and may be ofvarying lengths. The nanotubes may be conductive, semiconductive orcombinations thereof. Conductive SWNTs are useful in the manufacture ofnanotube films, articles and devices and can be used in the nanotubesolutions according to one or more embodiments of the invention. Thus,the nanotube composition is integratable into current electronicfabrication processes including, by way of example, CMOS,bipolar-transistor, advanced memory and logic device, interconnectdevice, and chemical and biological sensor fabrications.

In selecting a solvent for the nanotube composition, the intendedapplication for the nanotube composition is considered. The solventmeets or exceeds purity specifications required in the fabrication ofintended application. The semiconductor manufacturing industry demandsadherence to the specific standards set within the semiconductormanufacturing industry for ultra-clean, static-safe, controlled humiditystorage and processing environments. Many of the common nanotubehandling and processing procedures are simply incompatible with theindustry standards. Furthermore, process engineers resist tryingunfamiliar technologies. According to one aspect of the presentinvention, a solvent for use in a nanotube composition is selected basedupon its compatibility or compliance with the electronics and/orsemiconductor manufacturing industry standards.

Exemplary solvents that are compatible with many semiconductingfabrication processes, including but not limited to CMOS, bipolar,biCMOS, and MOSFET, include ethyl lactate, dimethyl sulfoxide (DMSO),monomethyl ether, 4-methyl -2 pentanone, N-methylpyrrolidone (NMP),t-butyl alcohol, methoxy propanol, propylene glycol, ethylene glycol,gamma butyrolactone, benzyl benzoate, salicyladehyde, tetramethylammonium hydroxide and esters of alpha-hydroxy carboxylic acids. In oneor more embodiments, the solvent is a non-halogen solvent, or it is anon-aqueous solvent, both of which are desired in certain electronicfabrication processes. In one or more embodiments, the solvent dispersesthe nanotubes to form a stable composition without the addition ofsurfactants or other surface-active agents.

In one aspect of the invention, nanotube compositions include aplurality of single-walled or multi-walled nanotubes in ethyl lactate asthe solvent. Ethyl lactate is one among the common solvent systems usedby the electronics and electronic packaging industry and is anindustry-accepted solvent that meets the industry standards for safetyand purity. Ethyl lactate is available as a high purity solvent, or itcan be purified to acceptable purity levels. Ethyl lactate hassurprisingly been shown to exhibit excellent solubilizing capabilitiesfor nanotubes. Furthermore, ethyl lactate can form stable nanotubecompositions even in the presence of significant levels of impurities,thereby providing a versatile solution for application for formation ofnanotube films and fabrics in a variety of applications. In one or moreembodiments of the present invention, a nanotube solution of SWNT inethyl lactate is provided. Purified SWNTs can be solubilized in ethyllactate at high concentrations, e.g., 100 mg/L, or even higher. Nanotubecompositions include nanotubes homogeneously distributed in ethyllactate without significant precipitation or flocculation.

Typical nanotube concentrations range from about 1 mg/L to 100 g/L, orfrom about 1 mg/L to 1 g/L, or about 10 mg/L, or about 100 mg/L, or evenabout 1000 mg/L with a common concentration used for memory and logicapplications of 100 mg/L. Such a concentration is exemplary varioususeful concentrations ranges depend upon the application. For example inthe case where a monolayer fabrics is desired one could use a lessconcentrated composition with a single or a few applications of thenanotube composition, e.g., by spin coating, to the substrate. In theevent that a thick multilayer fabric is desired, a spraying techniquecould be employed with a nearly saturated nanotube composition. Theconcentration is, of course, dependent upon the specific solvent choice,method of nanotube dispersion and type of nanotube used, e.g.,single-walled or multiwalled.

Nanotubes may be prepared using methods that are well known in the art,such as for example, chemical vapor deposition (CVD) or other vaporphase growth techniques (electric-arc discharge, laser ablation, etc.).Nanotubes of varying purity may also be purchased from several vendors,such as Carbon Nanotubes, Inc., Carbolex, Southwest Nanotechnologies,EliCarb, Nanocyl, Nanolabs, and BuckyUSA (a more complete list of carbonnanotube suppliers is found athttp://www.cus.cam.ac.uk/˜cs266/list.html). Vapor-phase catalysts aretypically used to synthesize nanotubes and, as a result, the nanotubesare contaminated with metallic impurities. Furthermore, formation ofnanotubes may also be accompanied by the formation of other carbonaceousmaterials, which are also a source of impurities in the nanotubes.

In one or more embodiments of the present invention, metallic particlesand amorphous carbon particles are separated from nanotubes. Thepurification process reduces alkali metal ions, halogen ions, oligomersor polymers as active or inactive chemical components as part of theSWNT solution. The nanotube solutions according to certain embodimentsof the present invention are substantially free of high levels of theseparticulate and/or insoluble materials (as well as other solublematerials that are incompatible with the semiconducting fabricationprocess). The nanotube solutions are thus purified for use in CMOSprocessing or other semiconducting fabrication process.

Appropriate purification techniques desirably remove impurities withoutaffecting the nanotube chemical structure or electronic properties.Impurities may be removed for example, by dispersing the nanotubes indilute acid solution to dissolve metal impurities, followed byseparation of the nanotubes from the metallic solution. A mild acidtreatment with nitric acid or hydrochloric acid may be used. Othersuitable methods for metal removal include magnetic purification.Amorphous carbon may be removed, for example, by a combination of highspeed centrifugation using an ultracentrifuge and filtration techniquesfor example but not limited to gravity filtration, cross flowfiltration, vacuum filtration and others. Other suitable purificationtechniques include the preferential oxidation of non-fullereniccarbonaceous materials. Multiple purification steps may be desired inorder to obtain nanotubes of a purity for use in a CMOS-grade nanotubesolution. See, for example, Chiang, et al., J. Phys. ChemB 105, 1157(2001); and Haddon, et al., MRS Bulletin, April 2004)

In one or more embodiments, nanotubes are pretreated to reduce themetallic impurity levels to preselected levels.

In one or more embodiments, the nanotubes composition contains less thanabout 10¹⁸ atoms/cm³ of metal impurities, or less than about 10¹⁶atoms/cm³ of metal impurities, or less than about 10¹⁴ atoms/cm³ ofmetal impurities, or less than about 10¹² atoms/cm³ of metal impurities,or less than about 10¹⁰ atoms/cm³ of metal impurities. Compositionshaving lower levels of metallic impurities, e.g. ca. 10¹⁰-10¹²atoms/cm³, may be used in the manufacture of advanced devices havingfine features, for example, devices having features of less than orequal to 250 nm.

Heavy metals, for examples metals having a specific gravity of 5 g/ml,are generally toxic in relatively low concentrations to plant and animallife and tend to accumulate in the food chain. Examples include lead,mercury, cadmium, chromium, and arsenic. Such compounds are carefullyregulated in the semiconductor fabrication industry and are desirablymaintained at minimum levels. In one or more embodiments, the nanotubecomposition includes less than about 10¹⁸ atoms/cm³ of heavy metalimpurities, or less than about 10¹⁶ atoms/cm³ of heavy metal impurities,or less than about 10¹⁴ atoms/cm³ of heavy metal impurities, or lessthan about 10¹² atoms/cm³ of heavy metal impurities or even less thanabout 15×10¹⁰ atoms/cm³ of heavy metal impurities.

Similarly, the concentration of group I and group II elements isregulated due to the deleterious effect of elements such as sodium,potassium, magnesium and calcium, and the like, on the performancecharacteristics of the electronic device. In one or more embodiments,the nanotube composition includes less than about 10¹⁸ atoms/cm³ ofgroup I and group II element impurities, or less than about 10¹⁶atoms/cm³ of group I and group II element impurities, or less than about10¹⁴ atoms/cm³ of group I and group II element impurities, or less thanabout 10¹² atoms/cm³ of group I and group II element impurities or evenless than about 15×10¹⁰ atoms/cm³ of group I and group II elementimpurities.

Lastly, transition metals are also avoided due to their ready migrationand the deleterious effect of such migration to the device performance.See, Mayer, et al. Electronic Materials Science: For Integrated Circuitsin Si and GaAs, 2nd Ed, Macmilliam, New York, 1988. As is the case forheavy metals and group I and group II metals, it is desired to maintainthe impurity level of transition metals, such as copper, iron, cobalt,molybdenum, titanium and nickel, to less than preselected values. In oneor more embodiments of the present invention, the nanotube compositionincludes less than about 10¹⁸ atoms/cm³ of transition metal impurities,or less than about 10¹⁶ atoms/cm³ of transition metal impurities, orless than about 10¹⁴ atoms/cm³ of transition metal impurities, or lessthan about 10¹² atoms/cm³ of transition metal impurities or even lessthan about 15×10¹⁰ atoms/cm³ of transition metal impurities.

The impurity content of the nanotubes can be monitored usingconventional methods, such as transmission electron microscopy (TEM) andscanning electron microscopy (SEM) and using analytical techniques suchas x-ray microanalysis (EDAX), or Vapor Phase Decomposition andInductively-Coupled Plasma Mass Spectrometry (VPD, ICP/MS).

Metallic impurity levels may be measured using conventional methods suchas EDAX and VPD, IPC/MS. If large quantities of solution (e.g., >about1000 mL), are available for testing, direct volumetric concentrationmeasurements (atoms/cm³) can be determined. Alternatively, a knownvolume of the composition may be deposited over a known surface area andthe surface impurity concentration (atoms/cm²) can be determined.

In other embodiments of the present invention, nanotubes are pretreatedto reduce the particulate impurities levels to a preselected level. Thesemiconductor industry has established standardized particulate impuritylevels for particular processes, and the nanotubes may be pretreated toreduce the nanotube particulate levels to below the accepted levels. Inone or more embodiments, the composition is substantially free ofparticle impurities having a diameter of greater than about 5 micron(μm), or about 1 μm, or about 3 μm, or about 500 nm, or 300 nm, or 100nm, or even 45 nm.

Guidelines for particulate and metal impurity levels are found in theInternational Technology Roadmad for Semiconductors (ITRS Roadmap). Forexample, the ITRS Roadmap states that at the 65 nm DRAM ½ pitch, thecritical particle size is 33 nm and only 1 particle/m³ is allowed overthe critical size. From the ITRS 2002 update, at the 90 nm DRAM ½ pitchnode, the critical particle size is 45 nm with only 2 particles/m³allowed above the critical particle dimension. The ITRS Roadmap for 90nmDRAM ½ pitch mode allows for <15×10¹⁰ atoms/cm3 of metal contaminationduring fabrication. Liquid chemicals utilized for wafer fabrication maycontribute <10 particles/mL of surface contamination. Other fabricationspecifications may be identified by the ITRS.

The semiconductor industry has well-established testing protocols formonitoring the particulate levels at, for example, 5 μm, 3 μm, 1 μm, 500nm, 300 nm and 100 nm. The metrology employed for detecting theparticulate contaminate will have a resolution of 0.2 nm. Typicalequipment include KLA Tencor surfscan™ and the like. Such testingmethods and equipment may be readily adapted for use in evaluating theparticulate levels of nanotube compositions.

In one or more embodiments of the present invention, the nanotubecomposition is a homogeneous mixture of purified single walled carbonnanotubes in ethyl lactate at concentrations high enough to be useful inpractical applications in the electronics industry, e.g., ≧10 mg/L. Thenanotube composition can be electronics-grade purity. In someembodiments, nanotubes purified to an impurity content of less than 0.2wt %, or less than 0.1 wt % free metal are solubilized inelectronics-grade ethyl lactate or other suitable solvent.

It has been surprisingly discovered that nanotubes that have beenpretreated to reduce the metallic and particulate impurity levels tobelow preselected levels, such as described herein, can form stablenanotube dispersions in a variety of solvents. Nanotubes, by way ofexample, SWNTs, and further by way of example purified SWNT, may besolubilized by dispersion in the appropriate solvent. One or more stepsof grind or agitating the nanotubes in the selected solvent andsonication may enhance solubilization.

The solution is appropriate for use as a spin-on SWNT solution forelectronic and electronic packaging applications. The inventors envisionthat the addition of various optional additives may be useful tofacilitate long term storage and stabilization properties of carbonnanotube solutions. Such additives include, but are not limited tostabilizers, surfactants and other chemicals known or accepted asadditives to solutions used for fabrication of electronics. The nanotubesolution according to one or more embodiments of the present inventionand the methods of making the solution of nanotubes have beenstandardized for CMOS compatibility as required in conventionalsemiconductor fabrication systems, i.e. the chemicals, spin coatingtracks and other related machineries necessary to create the solutionsof the present invention may be found in typical CMOS processingfacilities or more generally may be present in many types of servicescommon to the electronics industry including fabrication and packagingfacilities.

The nanotube composition can be placed or applied on a substrate toobtain a nanotube film, fabric or other article. A conductive articleincludes an aggregate of nanotubes (at least some of which areconductive), in which the nanotubes contact other nanotubes to define aplurality of conductive pathways in the article. The nanotube fabric orfilm desirably has a uniform porosity or density. In many applications,the nanotube fabric is a monolayer.

Many methods exist for the application procedure including spin coating,spray coating, dipping and many others known for dispersing solutionsonto substrates. For thicker fabrics beyond a monolayer, moreapplications or more concentrated solutions may be required. In factother techniques for application of the fabric may be required as hasbeen outlined elsewhere (See Nanotube Films and Articles (U.S. Pat. No.6,706,402) filed April 23, 2002 and Methods of Nanotube Films andArticles (U.S. patent application Ser. No. 10/128,117) filed Apr. 23,2002).

In one aspect of the invention, a highly purified nanotube article isprovided. The article contains a network of contacting nanotubes forform pathway through the article. The nanotube network may form a ribbonor non-woven fabric. The article contains less than 0.2 wt % or 0.1 wt %free metal, or even less.

In one or more embodiments, the nanotubes article contains less thanabout 10¹⁸ atoms/cm² of metal impurities, or less than about 10¹⁶atoms/cm² of metal impurities, or less than about 10¹⁴ atoms/cm² ofmetal impurities, or less than about 10¹² atoms/cm² of metal impurities,or less than about 10¹⁰ atoms/cm² of metal impurities. Compositionshaving lower levels of metallic impurities, e.g. ca. 10¹⁰-10¹²atoms/cm², may be used in the manufacture of advanced devices havingfine features, for example, devices having features of less than orequal to 250 nm.

Heavy metals, for examples metals having a specific gravity of 5 g/ml,are generally toxic in relatively low concentrations to plant and animallife and tend to accumulate in the food chain. Examples include lead,mercury, cadmium, chromium, and arsenic. Such compounds are carefullyregulated in the semiconductor fabrication industry and are desirablymaintained at minimum levels. In one or more embodiments, the nanotubearticle includes less than about 10¹⁸ atoms/cm² of heavy metalimpurities, or even less than about 15×10¹⁰ atoms/cm² of heavy metalimpurities.

Similarly, the concentration of group I and group II elements isregulated due to the deleterious effect of elements such as sodium,potassium, magnesium and calcium, and the like, on the performancecharacteristics of the electronic device. In one or more embodiments,the nanotube article includes less than about 10¹⁸ atoms/cm² of group Iand group II element impurities, or even less than about 15×10¹⁰atoms/cm² of group I and group II element impurities.

Lastly, transition metals are also avoided due to their ready migrationand the deleterious effect of such migration to the device performance.As is the case for heavy metals and group I and group II metals, it isdesired to maintain the impurity level of transition metals, such ascopper, iron, cobalt, molybdenum, titanium, and nickel, to less thanpreselected values. In one or more embodiments of the present invention,the nanotube article includes less than about 10¹⁸ atoms/cm² oftransition metal impurities, or even less than about 15×10¹⁰ atoms/cm²of transition metal impurities.

The use of the term “about” reflects the variation that occurs inmeasurement and can range up to 30% of the measured value. For example,when determining metal impurity levels using VPD ICP-MS, the accuracy ofthe measurement is related to the precision of analytical signals, therecovery of trace metals from the wafer surface, and the accuracy of thestandards used. Overall accuracy of the VPD ICP-MS technique varies from±15%, at concentration levels higher than 10 times above the methoddetection limit, to ±30% or higher at concentration levels lower than 10times the detection limits. Similar variability is expected in othermeasurements.

The following example are provided to illustrate the invention, which isnot intended to be limiting of the invention, the scope of which is setforth in the claims which follow.

EXAMPLE 1

This example describes the purification of nanotubes.

Single-walled carbon nanotubes (SWNTs) were purified by stirring in 7.7MHNO³ for 8 h followed by refluxing at 125° C. for 12 h. The acidrefluxed material was washed with DI water three times by asonication-centrifugation-decantation cycle. The DI water washedmaterial was then vacuum filtered over a 5 micron filter until a driedSWNT membrane was obtained on the filter paper. This purified SWNTmaterial was collected and used for making a SWNT composition.

EXAMPLE 2

This example describes the preparation of a nanotube composition and ananotube

In order to avoid recontamination of the nanotubes, clean roomconditions, for example, Class 100 or greater, were maintained duringpreparation and processing of the nanotube composition. Twenty-one mg ofsingle-walled nanotubes (SWNTs), purified as described above in Example1 were soaked in 10 mL ethyl lactate (electronics grade—Sigma), groundwith a mortar and pestle, sonicated and centrifuged to remove thesupernatant. These steps were repeated as necessary to solubilize thecarbon nanotubes. The solubilized nanotubes had a final concentration of21 mg carbon nanotubes per 250 mL ethyl lactate, and the optical densityat 550 nm of the solution was measured to be 1.001.

Each individual step of the solubilization process is detailed in theTable 1 for the solubilization of SWNTs in ethyl lactate (EL). Thisprotocol is illustrative of one means of forming a solubilized nanotubesolution. Many other methods of forming such a solution are possible byadding or subtracting steps involving agitation and solubilizationdepending upon the specific requirements for concentration, solutionstability and ultimate performance metrics of the desired fabric.

TABLE 1 Process Flow Chart for SWNT solubilization in Ethyl-Lactate StepProcess Duration Remarks 1 Soak in 10 ml EL 30 min In mortar 2 Grind 10min In mortar 3 Soak in 10 ml EL 1 h 20 min In mortar 4 Add 90 ml ELAfter transfer to 250 ml flask 5 Bath sonicate 0.5 h 5° C. 6 Centrifuge(10 krpm, 20° C.) 0.5 h In Teflon container 7 Decant supernatant Collectin 500 ml flask (100 ml); 25 C. 8 Grind sediment in 10 ml EL 10 min Inmortar 9 Soak 50 min In mortar 10 Add 90 ml EL After transfer to 250 mlflask 11 Bath sonicate 0.5 h 5° C. 12 Centrifuge (10 krpm, 20° C.) 0.5 hIn Teflon container 13 Decant supernatant Collect in 500 ml flask (200ml); 25° C. 14 Grind sediment in 10 ml EL 10 min In mortar 15 Soak 50min In mortar 16 Add 90 ml EL After transfer to 250 ml flask 17 Bathsonicate 0.5 h 5° C. 18 Centrifuge (10 krpm) 0.5 h In Teflon container19 Decant supernatant Collect in 500 ml flask (300 ml); 25° C. 20 Allowto stand 12 h At 25° C. in closed flask 21 Sonicate 1 h 5° C. 22 MetricsNA Check for sheet resistance and SEM 23 Storage conditions NA In 250 mlpolypropylene (PP) bottle; 5° C.

EXAMPLE 3

This example describes an alternative method of preparing a nanotubecomposition.

Twenty-one mg carbon nanotubes were mixed in 10 mL EL and subjected tosonication, centrifugation, decanting of the supernatant and remixing ofcarbon nanotubes in EL for repeated sonication until the tubes weresufficiently solubilized; i.e., the nanotubes were subjected essentiallythe same steps as in Example 2, without grinding with mortar and pestle.The steps of the process are shown in Table 2.

TABLE 2 Alternate Process Flow Chart for SWNT solubilization inEthyl-Lactate Step Process Duration Remarks 1 Place 100 mg in 800 ml ELN/A In 1 L polypropylene (PP) bottle. 2 Add Teflon impellers N/A In 1 LPP bottle 3 Place on autoshaker 100 h Powered through a timer 4 Collectin a 1 L RB N/A HF cleaned flask, in cleanroom 5 Bath sonicate 1 h 5° C.6 Centrifuge (15 krpm, 15° C.) 2 h 6 × 250; Beckman PP bottles 7 Decantsupernatant ~15 min Collect in 1000 ml flask 8 Check for optical densityat 550 N/A If above 1.25 this needs to be adjusted to nanometer. 1.25 byadding neat EL 9 Bath sonicate 2 h 5° C. 10 Centrifuge (25000 rpm, 15°C.) 2 h 8 × 50 cc, Beckman PP in 3 batches 12 Decant supernatant N/ACollect in 1000 ml flask (200 ml); 25° C. 13 Check for final metrics N/ABottled in a 1 L PP bottle rinsed with including sheet resistance andCMOS grade EL, SEM

EXAMPLE 4

This example describes the preparation of a nanotube article on asilicon substrate.

The solution prepared in Example 2 was spin coated onto a 100 mmoxide-coated silicon wafer. For comparison, a nanotube solution in ELusing as-prepared, i.e., unpurified, nanotubes was spin coated onto asimilar 100 mm oxide-coated silicon wafer. Six applications were used togenerate a fabric or film onto the wafer surface. FIGS. 1 and 3illustrate SEM images of unpurified SWNT material and purified SWNTmaterial, respectively coated from a solution of SWNTs in ethyl lactate.The presence of particulate impurities is apparent in the unpurifiedsample (FIG. 1).

The purified SWNT film showed significant reduction in amorphous carboncontamination after completion of the purification process (FIG. 2). Thefigures do not necessarily represent ideal electronics grade fabrics,but are shown simply to represent spun-on fabrics created from ethyllactate.

Upon generation of a fabric the sheet resistance was measured to be 70kOhm (center); 129+/−22 kOhm (edge). The following table (Table 3)summarizes several metric parameters including the optical density of atypical purified SWNT solution as well as the resistivity of a SWNTfabric on a 100 mm silicon wafer coated with a thick gate oxide.

TABLE 3 Metrics of Typical SWNT Fabric Metrics Data Remarks OpticalDensity 1.001 (550 nm) Sheet Resistance 70 kohm (center), 6 spins: 129+/− 22 kohm (edge) 60 rpm, 500 rpm, 4000 rpm

The solution can be used to form a component of NRAM memories, such asdescribed in U.S. patent application Ser. No. 09/915,093, entitled“Electromechanical Memory Array Using Nanotube Ribbons and Method forMaking Same”, filed Jul. 25, 2001; U.S. Pat. No. 6,643,165, entitled“Electromechanical Memory Having Cell Selection Circuitry Constructedwith Nanotube Technology,” filed Jul. 25, 2001; U.S. Provisional PatentApl. No. 60/459,223, entitled “NRAM Bit Selectable Two-Drive NanotubeArray,” filed Mar. 29, 2003; and U.S. Provisional Patent Appl. No.60/459,222, entitled “NRAM Byte/Block Released Bit Selectable One-DeviceNanotube Array,” filed Mar. 29, 2003. The solution holds potential as astand alone commercial product to serve the research and developmentlaboratories that work on single walled carbon nanotubes as well otherapplications.

EXAMPLE 5

This example describes the testing of trace metals on the surface of ananotube article that is deposited on a silicon wafer.

A nanotube composition was prepared from nanotubes that had beenpurified of metallic and particulate impurities as described in Example1 by dispersing the nanotubes in ethyl lactate medium as described inExample 2. The nanotube compositions were analyzed for surface metallicimpurities by Vapor Phase Decomposition and Inductively-Coupled PlasmaMass Spectrometry (VPD, ICP/MS) by Chemtrace, Fremont, Calif.

Silicon wafers, with and without a deposited nanotube layer, were placedin a pre-cleaned high purity chamber saturated with hydrofluoric acid(HF) vapor. Untreated silicon wafers and ethyl lactate coated waferswere used as controls. The native or thermal oxide on the silicon waferor deposited layer was dissolved in the presence of the HF vapor. Metalimpurities incorporated into the layer were released and dissolved inthe acid during the scanning process.

A drop of an ultrapure acid etchant is added to the surface and theanalysis area is scanned in a reproducible manner. The scanning solutionwas then collected for ICP-MS analysis. The analysis area was the entiresurface on one side of the wafer with 2 mm edge exclusion. Strictcleanroom practices were followed at all times. The VPD process wasperformed in a near Class 1 laminar flow mini-environment located in aClass 10 cleanroom. The ICP-MS instrument was operated in a Class 1000cleanroom to minimize environmental source contamination.

A pre-cleaned silicon wafer was used as the control. In order toevaluate the source of metallic impurities in the solvent, a siliconwafer was treated (spin-coated) with electronics grade ethyl lactatealone (EL Control). Samples 1 through 3 represent three differentnanotube compositions purified and prepared according to the methodologyset out in Examples 1 and 2. The test results demonstrate thatcomparable levels of purity were achieved over a number of samplestested. Most of the metals tested were near the detection limit of themethod. Notable exceptions to this were boron, calcium, cobalt, nickelpotassium and sodium. However, the total and individual metals contentwere well below the lower limit of 15×10¹⁰ atoms/cm³ set by the ITRS.Care must be taken in post purification processing in order to preservethe purity levels thus attained. For example, it was observed thatrinsing the as-deposited nanotubes with DI water reintroduced severalmetal impurities.

The results of trace metal analysis recording the elemental contentSWNTs after being coated on silicon substrates are reported in Table 4.Measurements are recorded as the number of atoms for a given element(×10¹⁰ atoms per cm²).

TABLE 4 (Number Of Atoms For A Given Element × 10¹⁰ Atoms Per cm²).Method Detection Limits Control EL Control Sample 1 Sample 2 Sample 3Aluminum (Al) 0.3 0.91 0.57 0.78 0.33 <0.3 Antimony (Sb) 0.003 <0.003<0.003 <0.003 <0.003 <0.003 Arsenic (As) 0.03 0.065 0.32 <0.03 <0.03<0.03 Barium (Ba) 0.01 <0.01 <0.01 <0.01 <0.01 0.01 Beryllium (Be) 0.1<0.1 <0.1 <0.1 <0.1 <0.1 Bismuth (Bi) 0.002 <0.002 <0.002 <0.002 <0.002<0.002 Boron (B) 1 140 220 5.7 5.9 5.3 Cadmium (Cd) 0.005 <0.005 <0.005<0.005 <0.005 <0.005 Calcium (Ca) 0.2 0.34 2.4 0.83 1.3 1.8 Chromium(Cr) 0.1 <0.1 0.11 <0.1 <0.1 <0.1 Cobalt (Co) 0.02 <0.02 <0.02 0.57 0.450.22 Copper (Cu) 0.05 <0.05 0.080 <0.05 0.34 <0.05 Gallium (Ga) 0.005<0.005 <0.005 <0.005 <0.005 <0.005 Germanium (Ge) 0.01 <0.01 <0.01 <0.01<0.01 <0.01 Iron (Fe) 0.1 <0.1 0.54 0.24 0.19 0.14 Lead (Pb) 0.003<0.003 0.012 <0.003 0.011 <0.003 Lithium (Li) 0.08 <0.08 <0.08 <0.08<0.08 <0.08 Magnesium (Mg) 0.3 <0.3 <0.3 <0.3 <0.3 <0.3 Manganese (Mn)0.03 <0.03 0.069 <0.03 <0.03 <0.03 Molybdenum (Mo) 0.01 <0.01 0.014<0.01 <0.01 <0.01 Nickel (Ni) 0.05 <0.05 <0.05 0.79 0.96 0.48 Potassium(K) 0.2 <0.2 3.5 0.30 1.2 0.73 Sodium (Na) 0.2 <0.2 7.1 1.2 2.1 1.5Strontium (Sr) 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Tin (Sn) 0.02 <0.02<0.02 <0.02 <0.02 <0.02 Titanium (Ti) 0.1 <0.1 <0.1 <0.1 <0.1 <0.1Tungsten (W) 0.005 <0.005 <0.005 <0.005 <0.005 <0.005 Vanadium (V) 0.03<0.03 <0.03 <0.03 <0.03 <0.03 Zinc (Zn) 0.06 <0.06 1.4 0.088 0.095 0.078Zirconium (Zr) 0.003 0.050 <0.003 <0.003 <0.003 <0.003

OTHER EMBODIMENTS

In certain embodiments concentrations of metallic or carbonaceouscontamination that are above those required for CMOS fabrication may beacceptable. The present invention serves to exemplify creation ofnanotube solutions with stringent requirements that meet or exceed thoseof a CMOS process flow but can be modified in applications that haverelaxed requirements.

In certain embodiments the SWNT solutions may be modified or tailored toform thick nanotube coatings up to 100 microns thick or more and as thinas a monolayer of SWNTs. Such nanotube fabrics can be characterized byresistivity or capacitance measurements to meet the requirements of thespecific electronics application.

As described herein, certain applicator liquids and applicationtechniques are described, which can be used to form nanotube films orfabrics of controlled properties. For example, certain proposals havebeen made suggesting the benefits of substantially monolayers ofnanotubes with substantially uniform porosity. Techniques have beenprovided in which one or more parameters may be controlled or monitoredto create such films. Moreover, these liquids are intended forindustrial environments, which require that the liquids be usable, i.e.,that the nanotube suspension is stable, for periods of days, weeks andeven months.

1. A spin-coatable liquid for formation of a nanotube film, comprising:a liquid medium containing a controlled concentration of purifiednanotubes, wherein the controlled concentration is sufficient to form ananotube fabric or film of preselected density and uniformity, andwherein the spin-coatable liquid comprises less than 1×10¹⁸ atoms/cm³ ofmetallic impurities.
 2. The spin-coatable liquid of claim 1, wherein thespin-coatable liquid comprises less than about 1×10¹⁵ atoms/cm³ ofmetallic impurities.
 3. The spin-coatable liquid of claim 1, wherein thespin-coatable liquid comprises less than about 15×10¹⁰ atoms/cm³ ofmetal impurities.
 4. The spin-coatable liquid of claim 1, wherein thespin-coatable liquid comprises less than about 1×10¹⁸ atoms/cm³ of heavymetal impurities.
 5. The spin-coatable liquid of claim 1, wherein thespin-coatable liquid comprises less than about 15×10¹⁰ atoms/cm³ ofheavy metal impurities.
 6. The spin-coatable liquid of claim 1, whereinthe spin-coatable liquid comprises less than about 1×10¹⁸ atoms/cm³ ofgroup I and group II element impurities.
 7. The spin-coatable liquid ofclaim 1, wherein the spin-coatable liquid comprises less than about15×10¹⁰ atoms/cm³ of group I and group II element impurities.
 8. Thespin-coatable liquid of claim 1, wherein the spin-coatable liquidcomprises less than about 1×10¹⁸ atoms/cm³ of transition metalimpurities.
 9. The spin-coatable liquid of claim 1, wherein thespin-coatable liquid comprises less than about 15×10¹⁰ atoms/cm³ oftransition metal impurities.
 10. The spin-coatable liquid of claim 1,wherein the spin-coatable liquid is substantially free of particleimpurities having a diameter of greater than about 500 nm.
 11. Thespin-coatable liquid of claim 1, wherein the spin-coatable liquid issubstantially free of particle impurities having a diameter of greaterthan about 300 nm.
 12. The spin-coatable liquid of claim 1, wherein thespin-coatable liquid is substantially free of particle impurities havinga diameter of greater than about 45 nm.
 13. The spin-coatable liquid ofclaim 1, wherein the nanotubes are homogeneously distributed in theliquid medium without substantial precipitation or flocculation.
 14. Thespin-coatable liquid of claim 1, wherein the liquid medium is anon-halogen solvent.
 15. The spin-coatable liquid of claim 1, whereinthe liquid medium is a non-aqueous solvent.
 16. The spin-coatable liquidof claim 1, wherein the liquid medium is selected for compatibility withan electronics manufacturing process.
 17. The spin-coatable liquid ofclaim 1, wherein the liquid medium comprises ethyl lactate.
 18. Thespin-coatable liquid of claim 1, wherein the spin-coatable liquid issurfactant-free.
 19. The spin-coatable liquid of claim 1, wherein thenanotubes comprise conductive nanotubes.
 20. The spin-coatable liquid ofclaim 1, wherein the nanotubes are single-walled nanotubes.